As is known in the art, a cruise control governor attempts to maintain a user-selected vehicle speed. Referring to FIG. 1(a), if the vehicle speed maintained by the cruise control governor is plotted as a function of time, it is apparent that the actual vehicle speed is not perfectly maintained at the cruise control set speed, because the controller can only attempt to maintain the desired set speed by measuring deviation of the actual speed from the set speed. The governor attempts to maintain a constant vehicle speed by controlling the amount of fuel which is provided to the engine, which is roughly proportional to the amount of torque that the engine will generate. FIG. 1(b) plots the engine torque vs. time which corresponds to the vehicle speed plot of FIG. 1(a). If the vehicle speed is plotted against engine torque, as in FIG. 2, a convenient paradigm is provided for visualizing the action of the cruise control governor. Viewing the cruise control governor from the perspective of FIG. 2 indicates that the engine will produce whatever engine torque is required to maintain a constant vehicle speed. Since the torque that goes into the vehicle varies with the terrain, the torque generation from the engine must also vary in order to maintain a constant vehicle speed.
Cruise control governors are devices that attempt to maintain a desired set speed condition by monitoring the system that they are trying to control. The cruise control governor monitors the road speed of the vehicle and reacts by changing the fuel command to the engine. For example, when the governor detects an underspeed condition, the governor increases the torque generation of the engine in order to increase the speed of the vehicle, thereby compensating for the undesirable underspeed situation. Thus, the governor is not capable of reacting until it recognizes that the vehicle has already deviated from the set speed. Once the vehicle has deviated from the set speed, it is too late for the governor to provide a perfect response, therefore the governor attempts to return the vehicle to the set speed as quickly as possible. Because the vehicle must deviate from the set speed before the governor reacts, it is impossible for the governor to provide a perfect response. This is why the plot of vehicle speed vs. time in FIG. 1(a) exhibits slight deviations both above and below the vehicle set speed. FIG. 3 is a process flow diagram which illustrates the interaction of the governor 22 with the vehicle/engine combination 24. The actual measured vehicle speed is subtracted from the desired set speed (which is set by the driver using the cab interface 20) in order to create a speed error signal. This speed error signal is input to the governor 22, which adjusts the fuel command signal to the vehicle/engine combination 24 in response thereto.
The plot of engine torque vs. vehicle speed in FIG. 2 is referred to as a "droop" curve. Such a droop curve is realized because the controller is attempting to follow a goal droop curve. The controller adjusts its response, and thus the response of the engine, as a function of the current operating conditions of the vehicle and as a function of the goal droop curve. FIGS. 4a-f illustrate examples of various goal droop curves. The shape of the goal droop curve used with any particular controller depends upon the particular response that is desired from the controller.
The ability for the controller to follow the goal droop curves depends upon the gain of the governor. The governor's gain is an indication of the aggressiveness of the controller. A high gain provides a very aggressive governor that will adjust engine torque generation rapidly in an attempt to follow the goal droop curve. However, aggressive gain governors also have a tendency to be unstable. In summary, the goal droop curves define where the controller attempts to maintain vehicle operation, and the governor gains define how aggressively the goal droop curves are followed.
Because vehicle speed determines where on the goal droop curve the controller attempts to operate, environmental factors which affect the speed of the vehicle affect the performance of the controller. One such environmental factor is the grade of the road surface upon which the vehicle travels. Gradability is a concept that allows one to consider the relationship between vehicle speed, the grade of a hill, the full torque curve of the engine, aerodynamic drag, gearing and torque requirements. This concept utilizes a grade curve as illustrated in FIG. 5. The grade curve denotes the torque needed, at every speed, to remain at an equilibrium for a certain combination of hill grade, aerodynamic drag, and gearing selection. FIG. 6 shows some examples of how various hill grades affect the placement of the grade curve. Such grade curves are useful because they provide an easy means to determine if the vehicle is going to accelerate or decelerate. If, at the current vehicle speed, the grade curve is higher than the torque curve, then the vehicle will slow down to the point of intersection between the grade curve and the torque curve. If, at the current vehicle speed, the grade curve is lower than the torque curve, then the vehicle will accelerate to a vehicle speed where the grade curve and the torque curve intersect. FIG. 7 shows an example of such movement.
When the vehicle goes over a hill, the grade varies depending upon where on the hill the vehicle is placed. FIG. 8 shows the various grades which are encountered by the vehicle on a symmetrical hill. As illustrated in FIG. 9, the grade curve for a vehicle progressing to the top of a hill will move to the left as the maximum percent grade is reached, and then move back to the right as the grade is decreased back to zero. If the vehicle slows down at all before the crest of the hill, due to the higher torque requirements, then the vehicle will accelerate before the top of the hill because the grade curve moves to the right as the vehicle approaches the crest of the hill (0% grade). The exact location of the start of the acceleration will depend upon the shape and length of the hill, the rating of the engine, and the aerodynamics of the vehicle.
Because most hills are relatively symmetrical and follow the model of FIG. 8, acceleration of the vehicle as it nears the crest of the hill is undesirable due to the fact that the vehicle will accelerate automatically on the downside of the hill due to the negative grade. Conversely, a vehicle entering a valley will decelerate on the downside of the hill prior to its eventual automatic deceleration when it encounters the upside of the hill on the opposite side of the valley. When a vehicle accelerates prior to a point where the terrain will cause the vehicle to accelerate automatically, or when a vehicle decelerates prior to a point where the terrain will cause the vehicle to decelerate automatically, fuel is wasted.
In the interest of increasing fuel economy of the vehicle, it is therefore desirable to design a controller which is able to recognize that the vehicle is cresting a hill or approaching the bottom of a valley and thereby alter the performance of the cruise control governor in order to obtain maximum fuel economy throughout the entire hill or valley event. The present invention is directed toward meeting these needs.